The present invention relates to a method and a device for determining the effect of the adjustment of actuators used in the manufacture of a web of material, and to a method of producing a web of material.
In the production of webs of material, particularly webs of paper or board, the cross section of the properties of the web of material and the sidewise offset of the web of material must be determined in order to be able, for the establishing of a desired cross section, to adjust certain actuators which are used in the manufacture of the web of material and which, are distributed over the width of the web, and affect the properties of the web of material.
It is known to remove portions of the web of material from the papermaking machine and examine them in the laboratory in order to determine the cross section of given properties of the web. This method is very expensive and has the disadvantage that undesired properties can be noted only very late. These properties can be avoided only much later, so that frequently large parts of webs of material are of defective quality or must be reprocessed as rejects.
In order to determine a transverse-shrinkage profile of a paper web it is furthermore known to apply markings to the web by suitable devices and to detect these markings by suitable devices, for example sensors (DE 40 08 282 A1). The paper machine must therefore be so designed that both the marking device and the suitable detection devices can be introduced into it. Frequently, the construction space necessary for this is not present or cannot be made available for reasons of space. Furthermore, the cost of such a paper machine is increased. The method for the determination of the transverse shrinkage is furthermore very expensive since additional marking means must be employed. Furthermore, marking is undesired since it reduces the quality of the paper.
Finally, in order to determine the local association of actuator positions with measurement positions, it is known to carry out so-called "bump tests" on the web of paper. These are test adjustments of individual actuators which are arranged sufficiently far apart, for the purpose of determining the places and geometrical shape of the effect of these actuators on the properties of the paper as a result of cross section measurements. Such test adjustments are then automatically carried out for example periodically or when desired by the user in order to recognize a change in the behavior of the process. The test adjustments must be so large that the result of the adjustment can be clearly noted in the paper and, after suitable filtration of the measured value, stands out from the process noise and measurement noise. The tests therefore interfere with the production process. During the bump tests and their evaluation it is not possible, of course, to act on the process at the same time by means of said actuators, so as, for instance, to compensate for existing disturbances in the process. Automatic control of the cross section of the properties of the web, if used, must be disconnected during the test. The accuracy with which the places of the effect of the adjusted actuators are determined is determined by the number of cross-section measurement values available, or by the distance between individual data values. This distance is ordinarily 1 cm to 10 cm. In any event, the accuracy obtained is insufficient to determine a precise transverse-shrinkage cross section from the places determined, as shown by the following illustrative calculation:
The places of the setting elements over the width of the production machine are precisely known. Two actuators which are as far apart as possible are adjusted. The distance x.sub.s between these actuators is known. The distance between the changes in section x.sub.p can be measured.
The percentage transverse shrinkage is then:
Shrinkage=((x.sub.s -x.sub.p)/x.sub.s).multidot.100%.
With a width of the paper web of, for instance, 5000 mm, a typical shrinkage of 5%, or 250 mm, and with a distance between measurement data of 25 mm, a determination of the total shrinkage which is sufficiently accurate is thus possible.
The object of the measurement is, however, to answer, for instance, the question as to at what place on the paper web the shrinkage is minimal, whether it is symmetrical, how the edges of the paper web behave as compared with the middle, etc. The shrinkage must therefore be measured as accurately as possible for the smallest possible regions on the paper web in order to obtain a meaningful transverse-shrinkage cross section.
In order, for instance, to be able to determine the shrinkage in a 500 mm wide region with an absolute precision of 0.5%, an accuracy of measurement in the paper of 500.times.0.005 mm=2.5 mm is necessary. With a distance between measurement data of 25 mm, this is not possible by far with simple bump tests.
The object of the invention is therefore to provide a method and a device for determining the effect of an adjustment of actuators as well as a method of manufacturing a web of material which does not have the said disadvantages.
This object is achieved by means of a method which can be referred to, from a control standpoint, broadly, as a "process monitor". The place where the adjustment of an actuator used in the manufacture of the web of material has its effect can be easily determined at any time without requiring the use of special marking devices or additional sensors which detect such markings. The production is not disturbed by test adjustments or by the disconnecting of an automatic cross-section control which may be present. By the iterative procedure and by the simultaneous consideration of many actuator adjustments and corresponding section measurements, a substantially more accurate association of the actuator positions with positions on the web of material is possible so that a meaningful shrinkage curve can be determined.
The mathematical determination of the effect is effected with the use of known values as to the behavior of the properties of the web of material. The mathematical determination of the effect is compared with measured values of a web-property cross section which have been actually obtained--before and after a actuator adjustment--in order to adapt the detection values in such a manner that the results of the calculation agree as closely as possible with the measured results as to the effect.
There is particularly preferred an embodiment of the method in which a prediction as to the place of the effect of the adjustment of the actuator is made. This method is characterized by the fact that it is relatively easy to carry out and can be optimized.
There is furthermore preferred a method which is characterized by the fact that a prediction is made as to the form of the effect, for instance as to the width of the effect at the place of the effect or the amplitude of the change in the properties of the web of material at the place of the effect. This method also is relatively simple to carry out and has the result that the prediction as to the place of the effect can be excellently optimized.
There is particularly preferred a method in which there are taken into account actuators which are in any event used for the manufacture of the web of material and are required in order to adjust the cross section of the properties of the web of material. Such a method is characterized by the fact that no additional devices or actuators are required which enlarge the construction space of the device for the manufacture of the web of material, increase the cost of construction and production, and possibly also require additional maintenance. Furthermore, the current manufacture of the web of material is not disturbed by an adjustment of the actuator which serves exclusively for purposes of measurement. The current production, therefore, remains unaffected.
There is particularly preferred a method in which the actuators are adjusted only to such an extent as is necessary during production in order to correct process disturbances by an automatic cross-section control. Larger adjustments would disturb the production and reduce the quality of the paper.
There is preferred a method in which the predictions are modified stepwise in the manner that first of all a general statement which is as general as possible with regard to the behavior of the properties of the web of material is made which is characterized in particular by the fact that it can still be made rapidly and with good precision even in the case of poor "signal-to-noise ratio".
One simple statement could, for instance, be the following: The edges of the web of material are displaced by a certain amount as compared with the position which they should have at the measurement point in the machine if there were no shrinkage and no sidewise offset. The two amounts of the adjustment are to be determined.
A first refinement of this statement would, for instance, be as follows: The transverse shrinkage is greater at the edges than in the middle and the transverse-shrinkage cross section resulting therefrom has a key-shaped contour the amplitude of which is to be determined.
It is therefore seen that by this method auxiliary devices for measuring the position of the edge-section devices and of the paper edges can be dispensed with.
Furthermore, a method is preferred in which the prediction as to the properties of the web of material comprises as accurate as possible a determination of the transverse-shrinkage behavior.
Finally, there is particularly preferred a method in which an averaging of the results in connection with the determination of the transverse-shrinkage cross section is carried out for a number of predictions and also for a number of actuator adjustments. Such an averaging has the result that the error upon the mathematical determination of the effect is reduced to a minimum.
The object indicated above is achieved by a device which has the at least one measuring device for detecting the cross section of a property of the web and an arithmetic unit. Due to the fact that the arithmetic unit processes predictions or detection values as to the behavior of the properties of the web of material upon the adjustment of an actuator and on basis of these detection values arithmetically determines the effect by means of a rule of association, it is possible, at any time, to predict on-line the place which is to be associated with the change of an actuator and to determine an exact cross section of a property of a web of material and/or determine a sidewise offset of the web.
The said object is finally achieved by a method of manufacturing a web of material from a water-pulp slurry in which at least one cross section property of the web is measured; and a plurality of actuators arranged spatially over the width of the web and influencing at least one of the properties of the web are adjusted in accordance with the measured values in such a manner that a desired cross section of a property of the web is established.